1. Field of the Invention
The field of this invention is high speed sample spinners, especially for nuclear magnetic resonance or ultra-centrifuge, employing hydrostatic gas bearings and radial-inflow microturbines.
2. Background of the Invention
Nuclear magnetic resonance (NMR) is a technique used to identify, compare and analyze molecular structures and compositions of materials. NMR is particularly difficult to apply to solid samples because of the effects of strong dipolar and higher order line broadening mechanisms from rigid neighboring nuclides. Many of the techniques developed for averaging out these effects include high speed spinning of the sample at various angles in the external magnetic field. A particularly lucid and eloquent statement of the various solids NMR techniques may be found in the opening discussion of U.S. Pat. No. 4,899,111. The primary object of this invention is the ability to achieve much higher spinning speeds at much greater stability and efficiency than previously could be obtained so as to permit improved analytical resolution under certain conditions.
Several spinners for nuclear magnetic resonance are well known. In U.S. Pat. No. 4,456,882 Doty discloses a technique for high speed sample spinning using cylindrical, ceramic sample containers with press-fit plastic turbines on hydrostatic air bearings. Several improvements are disclosed in U.S. Pat. No. 4,739,270 by Daugaard et al. A co-pending application, Ser. No. 07/607,521, discloses novel radial-inflow microturbines and other improvements appropriate for high temperature applications. Other small, high-speed, air bearings are disclosed in U.S. Pat. Nos. 3,969,822, and 4,366,993, and the references cited therein. Other NMR MAS spinners are disclosed in U.S. Pat. Nos. 4.254,373, 4,275,350, 4.511,841. 4,899,111 and the references cited therein.
Various techniques have been proposed to control whirl in gas bearings with varying degrees of success under various conditions. Some proposed techniques have probably contributed to increased whirl, such as the rotor depressions described in U.S. Pat. No. 4,521,190.
For larger bearings it is feasible to insert compliant metal foils, as disclosed by Warren, U.S. Pat. No. 4,552,466. and Sargent, U.S. Pat. No. 4,332,427. that create symmetric constriction zones, thereby largely eliminating the possibility of an asymmetric force due to viscous pumping of the gas by the rotating spindle. The overlapping-foil bearing is not only difficult to produce, it also has low stiffness and is ineffective at low speeds as it is hydrodynamic.
Herring-bone patterns in the bearing stator, as discussed by Yang and Munday in "A Grooved Self-Acting Beating for Use in Cryogenic Expansion Turbines", Cryogenic Processes and Equipment Vol. 79 (1982), have also been moderately effective, but they are extremely difficult to implement in small journal beatings. The balanced eccentric geometry of Romberg, U.S. Pat. No. 4,222,617, is also very difficult to manufacture, since tolerance requirements are typically under 1.5 .mu.m. Acoustically resonant chambers have been shown to be effective over a narrow speed range in the reference by Voth et al. but again they are difficult to apply to small systems. Converging-diverging shaft and bearing contours have been used by Miyake et al, U.S. Pat. No. 4,486,105, to reduce supersonic shock and improve stiffness at very high pressure ratios.
Radial-axial mixed-flow aluminum microturbines have been used as expansion machines in cryogenic refrigerators for three decades, and microturbines have been widely used by the dental industry for even longer. However, there is a need for a stable, high speed NMR sample spinner.